Magnetic hard disk drives continue to provide ever more storage space and faster access, as well as data transfer and retrieval times. Where once 100 kilobytes of stored data occupied a ten inch diameter footprint, 2.5 inch diameter disks now exceed 100 Gigabytes and information transfer and access times have undergone similar order-of-magnitude improvements.
One of the reasons for the continuing improvement is higher disk rotation speeds. Another reason is the continuing shrinkage of the size of a data bit recordable on the magnetic medium that coats the surface of a typical hard disk platter. The shrinking data bit footprint can be attributed to the ability to accurately position the reading and writing head close to the recording medium. The closer the head, the smaller the magnetic signal needed to define a recorded bit.
While there are mechanical and aerodynamic achievements behind the ability to get the read/write head to within microns of a recording surface, the electrical challenges have been no less daunting. Very precise control of write currents, for example, is necessary to write (record) a bit of uniform size to the medium. The uniform size of a bit is necessary in order to fit the highest possible number of bits onto a given platter surface size.
Access times, the time required to find a particular set of data bytes on the recorded surface, depend on the speed of disk rotation and on the speed with which the read/write head, mounted on the head arm, can be moved to a particular track on the platter. That head motion is easiest to attain with the lightest possible head and arm weights.
Conventional art FIG. 1A illustrates a typical hard disk drive. The recordable medium is coated on the surface of platters 101 where head arm 102 controls the position of read/write head 105. Read/write head 105 records and reads data written to the recordable medium. Write current, sufficient to magnetize the appropriate bit of recordable medium, is sent to the head from pre-amp/write driver 106. The position of head arm 102 is driven by arm actuator 103 under control logic from logic board 104. It is noted here that there are many existing configurations of hard disk drives with varying numbers of platters on a common spindle and an accompanying number of head arms, one for each recordable medium surface. It is also noted that, as the numbers of platters increases, the numbers of arms also increases, and the inertial load on the moving arm actuator increases.
Transfer time, the time required to read or write a given amount of data, depends on many elements, of which disk rotation speed is a significant part. Another significant contributor to transfer time in data writing is the speed with which write current to the write head can be switched on and off.
While higher power electronics in the head arm and read/write head can contribute to higher rates of data writing, higher powers typically entail larger and heavier components. A larger and heavier pre-amp/write driver (106 in FIG. 1) of which there is one for each head on each arm, would force an increase in the inertia of head arms 102, necessarily slowing access times.
It is noted here that write driver 106 is, in this illustration, mounted on head arm 102. The mounting location of Write driver 106 is the result of tradeoffs made in the design process. The farther the write driver is located from the write head, the more effect the impedance of the wires connecting the two will have on the write signal, thereby reducing its operational frequency. In general, shorter wires result in theoretically faster signals. However, the mass of the pre-amp/write driver contributes to the moment of inertia of the head arm. Generally, locating the pre-amp/write driver closer to the write head means locating it farther from the arm spindle 107 and vice versa. With more mass located farther from the spindle, the stronger, and heavier, the arm must be. The heavier the arm and the farther the pre-amp/write driver is from the arm spindle 107, or the heavier the pre-amp/write driver, the more the actuator energy required to move the arm from disk track to disk track rapidly and the higher demand on the arm actuator for precision control of the head arm.
Conventional art FIG. 1B illustrates a write driver circuit, in this case an “H-bridge” model of a typical write driver. An “X” logic signal or a “Y” logic signal, representing logic level commands for a logical “1” or “0” in the recorded medium, are sent to the driver from upstream logic. Amplifying transistors 111, 112, 113 and 114 are switched on or off as appropriate to control power supply voltage Vcc, 108, in the appropriate direction through the write head, represented in FIG. 1B by load resistances 115 and 116 and also load inductance 117. It is noted that the output load is approximated in this illustration. There are any number of different load models associated with existing write heads.
As demand for speed of writing to the recordable medium seeks ever faster write speeds, impedance in write drivers, connecting wires and in write heads limits switch-on, switch-off rates. A mismatch of impedances between the write driver and the write head, and associated wiring, often results in signal reflections and jitter which slows the attainment of the proper signal level to the write head. The proper write signal level is necessary to achieve a subsequently readable written bit that is also contained fully within the allowable bit footprint in the recorded medium.
Impedance mismatches occur because, when the one of the bottom transistors in the H-bridge, illustrated in FIG. 1B, is turned on by the appropriate logic input, the output impedance of the transistor becomes high and the signal is reflected at the output terminal. The output current thus has a large overshoot and ringing, or oscillation, and the overshoot and ringing, as discussed above, decrease the achievable data rate.
In order to match impedances as well as possible, drive designers choose from a selection of available pre-amp/write drivers, trading off between signal speed and quality and pre-amp/write driver location on the head arm. The design process can be iterative and slow and can result in compromises in component selection that result in non-optimum hard disk drive performance. Furthermore, maintaining a selection of components in order to accommodate differences between designs can be costly to the drive manufacturer.